THREE DECADES OF WIND TUNNEL INTEGRITY

The fan was constructed in 1984 and entered full service in 1986 but it wasn't until this year that the centre re-contacted James Walker RotaBolt to enquire about specialist lubrication greases, and to also report that the fasteners were still operating trouble-free in monitoring and maintaining the correct bolt load. The fan has a rotor diameter of 7.5 metres and operates at a speed of 350 RPM in a closed metal channel, creating an airflow of 55 m/s. There are eight blades which are 3 metres long and are each fastened to the main hub by ten M24 x 3.0 x 100mm RotaBolts which were installed with a pre-set load of 22 tonnes. The original specification of using RotaBolts over 30 years ago was made by the company that constructed the hand-crafted wooden blades – Permali Gloucester Ltd. Although they no longer manufacture these blades, they were able to confirm the original design parameters and detail of the fan's construction. Wind tunnel fan blades were generally constructed from laminations of selected spruce and birch woods. The aerofoil shape of the blades was hand crafted and each blade was attached into a forged machined steel adaptor using a custom tapered threaded root. The blades were then attached to the hub using the RotaBolts. The use of an adaptor allowed the blade pitch to be adjusted to alter the flow in the tunnel, unlike the fixed pitch blades which were simply sandwiched between two hub plates. The main characteristics of laminated wood blade construction are high structural integrity and resilience to operating stresses. Another key benefit is the freedom from fatigue – a quality also found in bolted joints that use RotaBolt measurement technology to successfully achieve and maintain the correct tension throughout their 30 year life.